Re: [PATCH RFC 0/8] virtio: add guest MSI-X support

Michael S. Tsirkin wrote:

On Mon, May 04, 2009 at 12:21:28PM +0300, Avi Kivity wrote:
  

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Michael S. Tsirkin wrote:
    

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So what I see is transports providing something like:

struct virtio_interrupt_mapping {
	int virtqueue;
	int interrupt;
};

map_vqs_to_interrupt(dev, struct virtio_interrupt_mapping *, int nvirtqueues);
unmap_vqs(dev);
        
          

Isn't that the same thing?  Please explain the flow.
    
        

So to map vq 0 to vector 0, vq 1 to vector 1 and vq 2 to vector 2 the driver would do:

struct virtio_interrupt_mapping mapping[3] = { {0, 0}, {1, 1}, {2, 2} };
vec = map_vqs_to_interrupt(dev, mapping, 3);
if (vec) {
  error handling
}

and then find_vq as usual.
  
      

Yes, that works.

Given that pci_enable_msix() can fail, we can put the retry loop in  
virtio-pci, and instead of a static mapping, supply a dynamic mapping:

   static void get_vq_interrupt(..., int nr_interrupts, int vq)
   {
       /* reserve interrupt 0 to config changes; round-robin vqs to  
interrupts */
       return 1 + (vq % (nr_interrupts - 1));
   }

   driver_init()
   {
       map_vqs_to_interrupt(dev, get_vq_interrupt);
   }

map_vqs_to_interrupts() would call get_vq_interrupt() for each vq,  
assuming the maximum nr_interrupts, and retry with smaller nr_interrupts  
on failure.
    

Since guest drivers are going to do round-robin most of the time, I
think the right thing to do is to make the API simple, along the lines
proposed by Rusty, and make the guest/host ABI rich enough to support
arbitrary mapping, along the lines proposed by you. We can always change
the API, ABI is harder.
  

We can provide the round-robin mapper as a helper, so driver code 
doesn't need to implement any callback if they're satisfied with the 
default.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.